1. Field
The following description relates generally to a rechargeable lithium battery.
2. Description of Related Art
Rechargeable lithium batteries have recently drawn attention as a power source for small portable electronic devices. They use an organic electrolyte solution and thereby may have twice the discharge voltage of a conventional battery using an alkali aqueous solution, and accordingly may have high energy density.
As for negative active materials of a rechargeable lithium battery, various carbon-based materials such as artificial graphite, natural graphite, and hard carbon, which all intercalate and deintercalate lithium ions, may be used. For positive active materials, composite metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCOxO2 (0<x<1), LiMnO2, and the like have been researched.
For an electrolyte solution, a lithium salt dissolved in a non-aqueous solvent of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and the like may be used.
During the initial charge of a rechargeable lithium battery, lithium ions, which are released from the lithium-transition metal oxide, i.e., the positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as a solid electrolyte interface (SEI) film.
The organic SEI film formed during the initial charge may prevent or reduce the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging. The organic SEI film may also act as an ion tunnel, allowing the passage of lithium ions. The ion tunnel may prevent or reduce disintegration of the structure of the carbon negative electrode, which may be caused by co-intercalation of organic solvents having a high molecular weight along with solvated lithium ions into the carbon negative electrode.
Once formed, the organic SEI film may prevent or reduce further reaction of lithium ions with the carbon electrode or other materials, and thereby reversibly maintain the amount of lithium ions. That is, carbon of the negative electrode reacts with an electrolyte during the initial charging, thus forming a passivation layer such as an organic SEI film on the surface of the negative electrode such that the electrolyte solution no longer decomposes, and stable charging and discharging is maintained.
Because of these reasons, in the rechargeable lithium battery, there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained. However, gases are generated inside the battery due to decomposition of a carbonate-based organic solvent during the organic SEI film-forming reaction. These gases may include H2, CO, CO2, CH4, C2H6, C3H8, O3H6, etc. depending on the type of non-aqueous organic solvent and negative active material used.
Due to the gases generated inside the battery, the battery may swell in a thickness direction when it is charged. When the battery is fully charged and kept at a high temperature (for example, 100% charged at 4.2V and allowed to stand at 85° C. for four days), the organic SEI film may gradually decompose based on the electrochemical energy and heat energy that increases as time passes, continuously causing a reaction with a new adjacent surface of the negative electrode when exposed with electrolyte solution.
The continuous generation of gases increases the internal pressure of the inside of the battery. There have been experiments for changing the organic SEI formation reaction by adding an additive to an electrolyte solution to suppress the internal pressure from increasing. However, when a particular chemical compound is added to an electrolyte solution to improve the battery performance, some aspects of the battery performance may be improved but some other aspects may be deteriorated.
For instance, when an additive is added to an electrolyte solution, the cycle life characteristic may be improved, but there may be a problem in that the thickness may be increased too much at a high temperature. To resolve the problem, fluoroethylene carbonate and/or vinylethylene carbonate have been used as additives to the solvent of an electrolyte solution.
However, this method does not resolve the swelling problem where the thickness of a battery is increased at a high temperature, when a nickel (Ni)-based positive active material, such as LiNiO2, especially a positive active material with some Ni replaced with cobalt (Co) or manganese (Mn), is used. Recently, research has been performed to replace a Co-based positive active material, such as LiCoO2, with the Ni-based positive active material.